In the TV answer back system of our U.S. Pat. No. 4,591,906 May 27, 1986 for Wireless Transmission from the Television Set to the Television Station, rf beep answers are transmitted by wireless transmission from receiver stations at very high frequenceis such as 53 megahertz or higher. In this system all receivers are operating on the same answer back frequency. It is imperative then that all the receivers have very carefully controlled answer back frequency to provide a very narrow bandwidth necessary for the communication channel, and to assure that all answers are received and processed at the TV studio. One means of control is to provide a temperature compensated oven for crystal controlled oscillators at each receiver station. However, for many thousand stations in the range of a single TV transmitter, this is an expensive and unreliable undertaking as shocks, aging, and other factors that affect oscillation frequency can not be controlled in a home environment.
Also, in a given geographical area, several TV signals from TV stations and cable channels may request answers simultaneously, and receiver stations with capability to answer back in two or more frequenceis may be desirable. Accordingly it is an objective of this invention to provide inexpensive and accurate control of oscillators located at receiver stations, in systems with single or multiple answer back frequencies.
The foregoing objective raises a serious problem, namely that a very high multiple megahertz oscillation frequency need be accurately frequency controlled without crystals in a home environment where temperatures, voltages and other operational conditions vary significantly, thereby tending to vary the oscillator frequency.
It is also desirable to have a frequency control system compatible with the technology used in the answer back equipment in order to further reduce cost. Thus, the use of pulse counting microprocessor technology is preferred. This then rises a problem that heretofore has not been resolved, namely it does not seem possible to control the megahertz oscillator frequency accurately within very few parts per million without having local means to control the environment (as in oven crystals), or to compensate for changes in ambient conditions (temperature compensation), or to adjust for changes due to aging (manual screw adjustments).
In a TV answer back system where TV stations encode query signals into the video signal, receiver stations send rf answer pulses in response to queries contained in the video signal, however, receiver stations will also send rf answer pulses in response to a video signal coming from a home video recorder when this signal was orginaly recorded from a TV broadcast containing encoded queries. This then raises a problem that heretofore has not been resolved, namely it does not seem possible to discriminate from a live TV signal containing queries and from a home video recording fo a TV signal containing queries, and thus prevent receiver stations from sending rf answer pulses that may interfere with answers from live broadcast queries.